Using conventional methodologies, a chip on wafer (CoW) bonding process involves the formation of copper (Cu) pillars on either the chip or the wafer, or both, before the chip is bonded to the wafer. If one of the chip and wafer does not include Cu pillars, it is instead provided with a Cu pad for bonding to the Cu pillars of the other of the chip and wafer. For simplicity, the following discussion will be limited to arrangements where there are pairs of opposing Cu pillars—i.e. the chip and wafer each have Cu pillars, with each Cu pillar of the chip aligning with a unique Cu pillar of the wafer—though the same discussion applies when bonding a Cu pillar to a Cu pad.
Each Cu pillar consists of a Cu post and a solder cap. The solder cap is typically applied using electroplating, followed by a reflow process to round the cap. During bonding, the solder caps of the chip melt and react with solder caps of the wafer to permanently fix the chip to the wafer.
The Cu pillars vary in height. This is the result of variance in the height of the Cu post and, or alternatively, the solder cap. As a consequence, thick solder caps are used, since solder deforms easily when melted, and pressure is usually applied to urge the chip against the wafer to ensure each Cu pillar meets and bonds with an opposing Cu pillar. This pressure can squeeze solder between the opposing Cu pillars. If the solder volume is large and the separation between Cu pillars (the “pitch”) is small, spreading solder can bridge neighbouring Cu pillars.
Despite this, there are many reasons why thick solder caps are considered unavoidable in conventional CoW processes. Flip chip bonding—a process that can permanently bond a chip to a wafer in a single step—requires thick solder caps to ensure there is sufficient solder on each of the chip and wafer so that opposing Cu pillars can bond through formation of an intermetallic alloy between the opposing pillars.
In addition, a thick solder cap is considered useful since the solder of a Cu pillar diffuses into the Cu post at elevated temperatures. This diffusion reduces the amount of available solder for the bonding process. Also, to form a smooth solder cap using a reflow process—the smoothness being necessary to ensure proper bonding and to reduce the impact of Cu pillar height variation—a relative thick minimum thickness is usually required.
It would be useful to provide a method that avoids the need for the thick solder caps, and thereby mitigates the potential for bridging.